I'm interested in finally taking the plunge into sours now that I have enough equipment and experience to not be totally scared to make one. I'd like to make a 1gal batch (potentially 2x 1 gallon batches) to get my feet wet with experimentation (and to take advantage of my glass jugs).

I'm interested in doing something that will leave some decent yeasty/bacterial goodness in the fermenter that I could then use to start a 5 gallon batch, so my initial 'cheater' berlinner weisse plan won't work since it gets boiled after the lacto does its work.

Am I going about this wrong to think that I could make a basic wort with 50%ish wheat and ~6 IBUs, split it into 2x 1-gallon batches, and pitch some stepped up dregs from something tasty into one and then some sort of blend into the other?

Sounds like you've got the right idea. For lambics, I typically use a 60/40 mix of wheat/2-row and less than 10IBU's from whatever hops I've had laying around for awhile. I usually let the hops sit out for a few weeks before using them.

I would suggest doing 5 gallons right from the start, then another 5 gallons 3-6 months later. Reason: If it turns out great...it's very hard to duplicate, and you'll be upset you didn't make more. If it turns out so-so, you'll have another batch to play with that you can blend.

I just got done bottling a Flanders red that's been aging on oak for the last 18 months. I added fresh sour cherries at the last 4 months. I'm so upset that I don't have more because it turned out fantastic.

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i agree you should go ahead and make 5 gallons i started with 2 gallons and went to a three gallon. i will be starting a 5 gallon batch friday but now realize i am a year behind where i would like to be

There's a rather long thread here about people doing one gallon batches. It's fine if you really don't know what you're going to get (e.g. spontaneous fermentation or souring a complex grain bill) or don't have a larger fermenter to give up for a year or more. If you're using a commercial strain or dregs and a basic recipe the risk of producing something bad is low enough you're better off making a larger batch.

Part of my desire to do smaller batches is equipment related - I can knock out a quick batch in all of the stuff that is 'mine' without having to run over to my brewing buddy's house to get the big kettle and remaining equipment. I'm currently doing small batches of cider-like stuff to pass the time and experiment. Plus i could go ahead and just get this started without feeling like I'm neglecting the other 2 batches worth of ingredients just waiting for a free evening to brew.

I do have a pretty beat-up carboy kicking around that is going to eventually be for sours, but I thought maybe I could have something from these small batches to use as a starter for a larger batch.

Do it. You have to start somewhere, and if you are ready for this, then do it rather than wait to do a bigger batch which might not happen for a while. I'd suggest you add some fresh sacc yeast with the dregs, and get the freshest dregs you can. It is likely the sacc yeast will be dead in the dregs due to age, but the Brett, Pedio, and Lacto will last very much longer. They too will eventually die, but you are looking at a few years before that happens.

Make the two single gallon batches and see where it goes. When you are ready to make a larger batch, all you need to do is rack the beer off one of these cakes, and add the cake to the larger batch. Keep these single gallon batches on the yeast cake (no secondary) until you have a need to use the cake.